Iron oxide pigments are becoming increasingly important due to the continuously increasing demand for environmentally harmless pigments. To qualify as environmentally harmless pigments, iron oxide pigments should be as far as possible free from trace elements.
Iron oxide black pigments used in the lacquer, plastics and building industry or as magnetic pigments slowly oxidize in the presence of oxygen or air.
As a result of this oxidation, they undergo an undesirable color shift to brown or red shades. In magnetic pigments, the oxidation causes a change in the magnetic properties.
Iron oxide black pigments are also required to be very stable at elevated temperatures, especially when they are used in stoving lacquers.
It is known from German 2,740,861=(U.S. Pat. No. 4,053,325) that iron oxide black pigments may be rendered heat resistant by a surface coating with up to 20% by weight of metal phosphates of the metals Al, Ba, Ca, Mg or Zn.
Such measures, however, are not acceptable where the intensity of color or the magnetic data of the pigment may be impaired by the colorless or non-magnetic coating or where very pure iron oxide black pigments free from coatings are required.
It is an object of the present invention to provide iron oxide black pigments which do not have the disadvantages described above.